原式=√y/(√2y-√y)-√y/(√2y+√y)
=√y/[√y(√2-1)]-√y/[√y(√2+1)]
=1/(√2-1)-1/(√2+1)
=(√2+1)/(√2+1)(√2-1)-(√2-1)/(√2+1)(√2-1)
=(√2+1)/(2-1)-(√2-1)/(2-1)
=√2+1-√2+1
=2